AB plays a central role in AD pathogenesis, although there is still a great need to fully define on which of the multiple AB assemblies underlay the neurotoxic properties and the amyloid-forming ability. While soluble oligomers more than fibrillar structures have been implicated in the mechanism(s) of neuronal toxicity, few in vivo experiments and our own recent data indicate that seeding of AB amyloidogenesis in APP transgenic mice can be accelerated by intracerebral infusion of soluble AD brain extracts but not to the same extent by homologous synthetic peptides in various states of aggregation, suggesting the existence of still undefined amyloidogenic co-factors. Two early-onset neurodegenerative conditions, familial British and Danish dementias (FBD and FDD) show extensive pre-amyloid and amyloid deposits, congophilic angiopathy and neurofibrillary tangle pathology, closely resembling AD. The deposited proteins (ABri in FBD and ADan in FDD), nevertheless, differ from AB in length and in primary structure and yet, all species share great propensity to oligomerize, assemble as ion-channel like structures in lipid bi-layers and form fibrils, all suggestive of common pathogenic pathways. We hypothesize that unrelated peptides could adopt similar altered amyloidogenic configurations that trigger comparable downstream detrimental effects in neuronal cells, being capable of accelerating amyloid deposition in vivo when in conjunction with additional amyloidogenic co-factors. Accordingly, we propose: Aim 1: to compare oligomeric and fibrillar ABri, ADan and AB assemblies isolated from brains with FBD / FDD / AD as well as from Tg mouse models and determine the structural requirements for synthetic ABri, ADan and AB to form in vitro similar assemblies to those found in vivo; Aim 2: to test the functional effect of the oligomeric and fibrillar ABri and ADan species characterized in aim 1 in comparison with analogous AB structures on their differential neurotoxic potential, assessing the induction of specific cell-death mechanisms (activation of initiator and effector caspases, mitochondrial pathways, Ca2+ dysregulation, etc) in ex vivo and in vitro paradigms, validating the results with tissue fractions enriched in oligomeric assemblies as well as synthetic homologues reconstituted in amyloid-depleted tissue extracts; Aim 3: to (a) analyze the exogenous amyloidogenic capability of oligomeric/fibrillar AB, ABri and ADan species in vivo in APP and ADanPP Tg animals using intra-hippocampal injections of brain extracts compared with synthetic homologues with analogous oligomerization in the presence/absence of amyloid-depleted brain extracts and (b) conduct a comparative proteomic analysis of the amyloid-inducing extracts to identify common post-translational modifications and/or additional co-factor(s) responsible for the amyloid-inducing activity.